* He can use it to embarrass, harass, or blackmail you. Want to ruin someone's reputation in a hurry? Log on as them and post humiliating or hateful content on their page.
But here's the bigger threat. Facebook really wants to be the single sign-on engine for the Web (see "What's to like about Facebook's 'Like' button?"). So a Facebook log on isn't just a Facebook log on anymore; it's also a log on to sites like Unvarnished, the Huffington Post, and any others that use Facebook Connect. If that's not an argument against using Facebook for single sign on, I don't know what is. Even if you don't use FB Connect, most people tend to use the same log ons for multiple sites; once a crook has your email address and favorite password, he can go to town on you. Nervous yet?
Bottom line: Your Facebook credentials are important, and only getting more so. If you want to protect yourself online, you'll need to protect them as well. Start by mixing up your passwords for your favorite sites, changing them semi regularly, and not getting duped by every stupid email marked "urgent."
Also: I gotta say I find this whole thing kind of insulting. I've got 700+ friends. I think my account is worth at least $1.50. Don't you?
When not not-getting duped, author Dan Tynan gets dopey at eSarcasm, his humor site for geeks who are not easily offended.